The nice thing about the DF raws is that they now list the actual species used as a basis. A lot of research went on some time ago to add correct, sourced information to the raws. There are probably still a few wonky things, as some of the properties in there make more sense for metals than things like wood, but the players care about even strange details like that.
Some fans decided to try and get accurate densities for as many materials as possible so that their in-game simulations reflect their actual properties.
The other reply linked to the forum thread, but just to elaborate, fans of the game obtained a box of Saguaro wood, then proceeded to measure the solid density and reported their findings.
That said, what actually prompted the raws to get updated was this: http://imgur.com/EPtgt
And the jokes on that page will only make sense if you play DF.
One more bit of background that may help you make sense of this. In DF, material properties, among other things, are defined in the "raws" (text files that contain assorted figures, like solid density).
Density was determined experimentally. Contact Uristocrat for a sample if you want to verify this yourself. A 6g (+/- 0.1g) piece of dry Saguaro wood had a volume of approximately 14 cm^3 (+/- 1 cm^3) [SOLID_DENSITY:430]
It's ~430 kg/m^3 dry and this was determined empirically by players.
EDIT: Fixed units.